1. Field of the Invention
The present invention relates to a linear motor apparatus suitable for use in office automation equipment, machine tools, semiconductor exposure systems and the like.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 64487/1993 titled "A Positioning Table Apparatus" discloses the operation of a motor only during an initialization operation. However, even in a normal operating state, coils are excited phase by phase successively to generate propulsion.
FIG. 3 is a block diagram showing the concept of an example of a conventional propulsion control method of a linear motor apparatus. In FIG. 3, a mover 1 of the linear motor has a plurality of N poles and S poles (mover permanent magnets 2a to 2d) of a permanent magnet to form a magnetic flux distribution thereon. A stator 3 of the linear motor has a plurality of stator coils 4a to 4f aligned in a direction at right angles to the direction of the magnetic flux of the mover 1. A position detecting means 5 measures the relative position of the mover 1 with respect to the stator 3 and the distance between the mover 1 and the stator 3. The operation of this system will be described briefly.
The position detecting means 5 outputs a relative position signal 6 representing the relative position of the mover 1 with respect to the stator 3 and the distance between the mover 1 and the stator 3. By the relative position signal 6, a target current value generation means 9 determines a single coil to be energized by the procedures shown in a flowchart of FIG. 5, and outputs currents, which are proportional to a target propulsion signal 8 generated by target propulsion generation means 7 as target current values 10a to 10f, to the coil. Power supply means 11a to 11f constitute, for example, a current minor loop to supply power so that the target current values can be actually passed through the selected coil. The supply of ordered currents 12a to 12f to the specified coil generates a target propulsion on the mover 1.
The flowchart of FIG. 5 showing the procedures of the target current value generation means 9 will now be described. The flowchart is described by means of an equation for determining the propulsion of the linear motor. The construction of this type of linear motor is shown in FIG. 4. In FIG. 4, the left end of the stator 3 is taken as the coordinate origin, and a coordinate along the length of the stator 3 is taken as an x coordinate. The position of the mover 1 can be represented by the x coordinate i.e., the distance from the origin to the left end of the mover 1. In the mover 1, the left end of the mover is taken as the origin and the right side thereof is in a plus direction and taken as .tau.. The positions of the coils aligned on the stator 3 are represented by P.sup.L.sub.k, and P.sup.R.sub.k in which k is a number of identifying the coil, the numbering starting from the leftward end of the stator 3; R and L denote the right end and left end of each coil, respectively; and i.sub.k is a current passing through the coil k. When a function U(x) represented by the following equation (1) is considered, a current i(y) at the y coordinate on the stator 3 (a certain distance from the coordinate origin in the same direction as the x coordinate) is represented by the following equation (2): ##EQU1## where D is the effective length of each coil.
Therefore, the propulsion F(x) which acts on the mover 1 is determined by the following equation (3): ##EQU2##
where B is a magnetic flux distribution, D is the effective length of each coil and Ly is a length of the stator 3. In this way, the propulsion force acting on the mover 1 is represented by the sum total of the products of distributed magnetic fluxes and currents which interlink with the magnetic fluxes.
Assuming that the distribution of current supplied to each coil is a point, the equation (3) can be transformed into the following equation (4): ##EQU3##
Taking the width of each coil as a fixed value L, the pitch between coils as L/2 and a target current value as i.sub.r, the exciting current of the conventional propulsion control method can be determined by the following equations (5): ##EQU4##
In the flowchart of FIG. 5, the above-described relational equation is executed. That is, means 9 determines whether the left end of the coils are in turn, positioned directly under the mover 1. When the coils are positioned directly under the mover 1, the coil in the position where the magnetic flux distribution is constant is determined from the left end position of the coil so as to generate a current command value such that the magnitude of the current supplied to the coil is proportional to a target propulsion command value, and the direction of propulsion matches the sign of the current. The position of the mover 1 in which the magnetic flux distribution is fixed is a section in which the amplitude becomes 1/.sqroot.2 on the assumption that the magnetic flux distribution is a sine wave of the amplitude of 1.
The method shown in FIG. 5 will now be discussed. In step SA1 the coil number, k, is set to 1. In step SA2, the position under the mover 1, S, of a coil to be energized is determined by the formula S=P.sup.L.sub.K -x. In step SA3, the target current value generation means 3 determines whether S is greater than or equal to 0. If not, this means that the left end, x, of the mover 1, is to the right of the left end of the coil k because P.sup.L.sub.K is less than x. In this case, the method skips to step SA16. If S is greater than or equal to 0, the method proceeds to step SA4, where the means 9 determines whether S is greater than or equal to L/4. If not, the method proceeds to step SA5, where means 9 determines the value of the exciting current to be -i.sub.r for the second coil from the left, since i.sub.k+1 =i.sub.1+1 =i.sub.2. Next, in step SA6, i.sub.k is set to 0 and k is incremented by 1 to k+1. Then, in step SA7, k is incremented again by 1. In step SA8, means 9 determines whether k is greater than N, the number of the coils. If k is greater than N, the process is completed. If k is not greater than N, i.sub.k is set to 0 in step SA9 and the method returns to step SA8.
In step SA4, if S is greater than or equal to L/4, the method proceeds to step SA10 where means 9 determines whether S is greater than or equal to 3/4L. If not, the method proceeds to step SA11 where i.sub.k (the exciting current for the k-th coil) is set equal to i.sub.r, (the target current) and the method proceeds to step SA7. If so, the method proceeds to step SA12, where means 9 determines whether S is greater than or equal to 5/4L. If not, the exciting current for the k+1-th coil is set to i.sub.r in step SA13, and the method proceeds to step SA6. If so, the method proceeds to step SA14, where means 9 determines whether S is greater than or equal to 6/4L. If not, the method proceeds to step SA15 where the exciting current for the k-th coil is set to -i.sub.r, and the method then proceeds to step SA7. If so, the method proceeds to step SA16 where means 9 sets i.sub.k to 0 and increments k to k+1. Then, the method proceeds to step SA17 where means 9 determines whether k is greater than N. If k is greater than N, the method is completed. If k is not greater than N, the method returns to step SA2.
According to the conventional method, only one coil to be excited is determined by the position of the mover, and the magnitude of the current supplied to the coil is set to a value proportional to the target propulsion, whereby the propulsion generated in the mover is controlled to a desired value.
As described above, according to the conventional method, the propulsion generated in the mover is controlled by utilizing a substantially fixed portion of the magnetic flux distribution produced by a permanent magnet. With respect to a change in the magnetic flux distribution due to a shifting of the mover, the propulsion generated is controlled so as to be proportional to the target current value by switching the coils which are successively excited. However, the magnetic flux distribution produced by the permanent magnet cannot be completely fixed even if a specific portion thereof is utilized. For this reason, a propulsion instability due to a nonuniform magnetic flux distribution is produced. In addition, in switching of the exciting coils, instability is also produced in the propulsion generated when the current value is not zero.
This state is shown in FIGS. 6A and 6B. FIG. 6B illustrates the propulsion generated in the mover when the target propulsion is fixed (44[N]). FIG. 6A illustrates values of command currents 12b to 12e to each of coil. In this way, the conventional method produces propulsion instability due to the non-uniformity of the magnetic flux distribution and propulsion instability produced in switching the exciting coils. Particularly, since the propulsion generated when switching the coils appears as vibration having a high frequency component, it exerts an adverse effect (such as a large vibration) when a speed control device and a position control device are constructed with the use of this propulsion control device.